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   Meet-in-the-Filter and Dynamic Counting with Applications to SpeckBiryukov, Alexei  ; Cardoso Dos Santos, Luan ; Teh, Je Sen et alin Tibouchi, Mehdi; Wang, Xiaofeng (Eds.) Applied Cryptography and Network Security. 20th International Conference, ACNS 2022, Rome, Italy, June 20–23, 2022, Proceedings (in press) We propose a new cryptanalytic tool for differential cryptanalysis, called meet-in-the-filter (MiF). It is suitable for ciphers with a slow or incomplete diffusion layer such as the ones based on Addition ... [more ▼] We propose a new cryptanalytic tool for differential cryptanalysis, called meet-in-the-filter (MiF). It is suitable for ciphers with a slow or incomplete diffusion layer such as the ones based on Addition-Rotation-XOR (ARX). The main idea of the MiF technique is to stop the difference propagation earlier in the cipher, allowing to use differentials with higher probability. This comes at the expense of a deeper analysis phase in the bottom rounds possible due to the slow diffusion of the target cipher. The MiF technique uses a meet-in-the-middle matching to construct differential trails connecting the differential’s output and the ciphertext difference. The proposed trails are used in the key recovery procedure, reducing time complexity and allowing flexible time-data trade-offs. In addition, we show how to combine MiF with a dynamic counting technique for key recovery. We illustrate MiF in practice by reporting improved attacks on the ARXbased family of block ciphers Speck. We improve the time complexities of the best known attacks up to 15 rounds of Speck32 and 20 rounds of Speck64/128. Notably, our new attack on 11 rounds of Speck32 has practical analysis and data complexities of 224.66 and 226.70 respectively, and was experimentally verified, recovering the master key in a matter of seconds. It significantly improves the previous deep learning-based attack by Gohr from CRYPTO 2019, which has time complexity 238. As an important milestone, our conventional cryptanalysis method sets a new high benchmark to beat for cryptanalysis relying on machine learning. [less ▲] Detailed reference viewed: 30 (0 UL) Advancing the Meet-in-the-Filter Technique: Applications to CHAM and KATANBiryukov, Alexei ; Teh, Je Sen ; Udovenko, Aleksei in Smith, Benjamin; Wu, Huapeng (Eds.) Selected Areas in Cryptography (in press) Recently, Biryukov et al. presented a new technique for key recovery in differential cryptanalysis, called meet-in-the-filter (MiF). In this work, we develop theoretical and practical aspects of the ... [more ▼] Recently, Biryukov et al. presented a new technique for key recovery in differential cryptanalysis, called meet-in-the-filter (MiF). In this work, we develop theoretical and practical aspects of the technique, which helps understanding and simplifies application. In particular, we show bounds on MiF complexity and conditions when the MiF-enhanced attack may reach them. We present a method based on trail counting which allows to estimate filtering strength of involved rounds and perform consequent complexity analysis with pen and paper, compared to the computer-aided approach of the original work. Furthermore, we show how MiF can be combined with plaintext structures for linear key schedules, allowing to increase the number of attacked rounds or to reduce the data complexity. We illustrate our methods on block cipher families CHAM and KATAN and show best-to-date single-key differential attacks for these ciphers. [less ▲] Detailed reference viewed: 45 (7 UL) |
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